Research News

How yeast makes heads or tails of itself

Colonies of yeast (Saccharomyces cerevisiae) pictured under a microscope. In glucose-rich conditions (left), yeast cells grow in a tight cluster. But when glucose is limited (right), new cells grow outward, forming a filament-like configuration that may aid in the search for food. A new study identifies the cellular mechanisms that cause the change in growth pattern, as well as the amount of time it takes for the shift to occur. Bar, 10 microns. Image: Paul J. Cullen

By CHARLOTTE HSU

“How do organisms determine what their front and back ends are doing when things are changing? How, for example, does a cell know its head from its tail when it’s dividing? We look at this question in relation to yeast.”

Paul Cullen, associate professor

Department of Biological Sciences

Yeast has been a friend to humanity since ancient times, when
people first learned to harness the organism to make bread and brew
beer.

Yet, we don’t always think of yeast as something
remarkable. Instead, it’s often perceived as plain or dull
— a single-celled organism that, like a plant, lacks the
ability to move on its own accord.

But even the simplest creatures have their wonders, as a new
piece of science shows.

Led by UB biologist Paul J. Cullen, the research was published
yesterday in the Early Edition of The Proceedings of the National
Academy of Sciences. It highlights the remarkable adaptability of
Saccharomyces cerevisiae, common baker’s yeast.

Researchers previously observed that the species, which lives in
colonies, can alter its pattern of growth in response to food
scarcity. Starve a colony of S. cerevisiae and new cells
start growing outward in a filament-like configuration that may aid
the search for nutrients.

The new study identifies the amount of time it takes for this to
happen — within 2 1/2 hours, affecting the very next
generation of yeast cells born in the colony — and also
identifies the mechanism through which the changes occur.

The shift in growth patterns has to do with the mysterious
concept of proprioception. In people, proprioception is like a
“sixth sense” that gives us an understanding of our
position and movement, says Cullen, associate professor of
biological sciences in the College of Arts and Sciences. But how do
cells “sense” their spatial orientation? That’s
the question the study addresses.

“All organisms have a sense of orientation,” Cullen
says. “Humans have a head and feet. In the stomach, cells
have one side that faces inward toward the body and another that
faces out toward the stomach acid, and these two sides are very
different.

“All organisms have this duality, called polarity, and
it’s really important for animals and cells to do what they
do. But what’s less clear is how we sense our orientation
during day-to-day decision-making. How do organisms determine what
their front and back ends are doing when things are changing? How,
for example, does a cell know its head from its tail when
it’s dividing? We look at this question in relation to
yeast.”

A yeast’s sense of self

Colonies of yeast (Saccharomyces cerevisiae) pictured under a microscope. In glucose-rich conditions (inset), yeast cells grow in a tight cluster. But when glucose is limited (main image), new cells grow outward, forming a filament-like configuration that may aid in the search for food. A new study identifies the cellular mechanisms that cause the change in growth pattern, as well as the amount of time it takes for the shift to occur. Bar, 10 microns. Image: Paul J. Cullen

In yeast, the heads-or-tails phenomenon that Cullen describes
involves the direction in which a colony spawns new cells.

Under normal circumstances, existing yeast cells — called
mother cells — give rise to daughter cells that grow from the
end of the cell that faces the colony. The resulting colony
resembles a ball.

But when nutrients are scarce, this pattern reverses itself
within a single generation, the study finds. During times of
starvation, daughter cells start budding from the opposite end of
the mother, growing away from the heart of the colony in a
filament-like pattern.

Through painstaking microscopy, Cullen’s team showed this
change occurred within 2 1/2 hours — the time it takes for a
yeast cell to make just one division. The work involved viewing
small groups of yeast cells repeatedly under a microscope and
logging their behavior over time.

Additional analyses enabled the scientists to understand what
happened inside the cells to jumpstart the new growth pattern.

The researchers linked the change to proteins found on opposite
ends of a yeast cell. The side of a yeast cell that faces its
mother contains axial bud proteins, while the other side contains
the Bud8 protein. When nutrients are high, axial bud proteins give
off chemical signals that activate complex decision-making pathways
within individual yeast cells. These pathways direct the cells to
produce daughters facing the colony.

But when nutrients are low, Bud8 churns into gear and gives
different instructions. It activates decision-making pathways that
tell cells to produce daughters from the opposite end — the
Bud8 end — which faces outward, away from the colony.

The team discovered, in essence, how yeast cells decide which
way to orient themselves — in other words, how these tiny
single-celled organisms get their bearings.

“What we see is that a yeast’s polarity — the
direction in which it grows — can be attributed to the
instructions it receives from Bud proteins, these spatial
landmarks,” Cullen says. “The Bud proteins give the
cell information about what orientation it should be in, about
which direction to grow. When there are lots of nutrients,
there’s one decision-making process that comes into play, and
when nutrients are scarce, there’s another. So these spatial
marks help cells make decisions during filament
formation.”